One class of opto-electrical devices is those using an organic material for light emission or detection. The basic structure of these devices is a light emissive organic layer, for instance a film of a poly(p-phenylenevinylene) (“PPV”) or polyfluorene, sandwiched between a cathode for injecting negative charge carriers (electrons) and an anode for injecting positive charge carriers (holes) into the organic layer. The electrons and holes combine in the organic layer generating photons. In WO 90/13148 the organic light-emissive material is a polymer. In U.S. Pat. No. 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as (8-hydroxyquinoline) aluminium (“Alq3”). In a practical device one of the electrodes is transparent, to allow the photons to escape the device.
A typical organic light-emissive device (“OLED”) is fabricated on a glass or plastic substrate coated with a transparent first electrode such as indium-tin-oxide (“ITO”). A layer of a thin film of at least one electroluminescent organic material covers the first electrode. Finally, a cathode covers the layer of electroluminescent organic material. The cathode is typically a metal or alloy and may comprise a single layer, such as aluminium, or a plurality of layers such as calcium and aluminium. Other layers can be added to the device, for example to improve charge injection from the electrodes to the electroluminescent material. For example, a hole injection layer such as poly (ethylene dioxythiophene)/polystyrene sulfonate (PEDOT-PSS) or polyaniline may be provided between the anode and the electroluminescent material. When a voltage is applied between the electrodes from a power supply one of the electrodes acts as a cathode and the other as an anode. For organic semiconductors important characteristics are the binding energies, measured with respect to the vacuum level of the electronic energy levels, particularly the “highest occupied molecular orbital” (HOMO) and the “lowest unoccupied molecular orbital” (LUMO) level. These can be estimated from measurements of photoemission and particularly measurements of the electrochemical potentials for oxidation and reduction. It is well understood in this field that such energies are affected by a number of factors, such as the local environment near an interface, and the point on the curve (peak) from which the value is determined. Accordingly, the use of such values is indicative rather than quantitative.
In operation, holes are injected into the device through the anode and electrons are injected into the device through the cathode. The holes and electrons combine in the organic electroluminescent layer to form an exciton which then undergoes radiative decay to give light. One way of improving efficiency of devices is to provide hole and electron transporting materials—for example, WO 99/48610 discloses blending of hole transporting polymers, electron transporting polymers and electroluminescent polymers. A 1:1 copolymer of dioctylfluorene and triphenylamine is used as the hole transporting polymer in this document.
A focus in the field of polymer OLEDs is the development of full colour displays for which red, green and blue emissive materials are required. One drawback with existing polymer OLED displays relevant to this development is the relatively short lifetime of blue emissive materials known to date (by “lifetime” is meant the time for the brightness of the OLED to halve at constant current when operated under DC drive).
In one approach, the lifetime of the emissive material may be extended by optimisation of the OLED architecture; for example lifetime of the blue material may in part be dependant on the cathode being used. However, the advantage of selecting a cathode that improves blue lifetime may be offset by disadvantageous effects of the cathode on performance of red and green materials. For example, Synthetic Metals 111-112 (2000), 125-128 discloses a full colour display wherein the cathode is LiF/Ca/Al. The present inventors have found that this cathode is particularly efficacious with respect to the blue emissive material but shows poor performance with respect to green and, especially, red emitters.
Another approach is development of novel blue electroluminescent materials. For example, WO 00/55927, which is a development of WO 99/48160, discloses a blue electroluminescent polymer of formula (w):

wherein w+x+y=1, w≧0.5, 0≦x+y≦0.5, and n≧2
In essence, the repeat units of the separate polymers disclosed in WO 99/48160 are combined into a single molecule. The F8 repeat unit is provided for the purpose of electron injection; the TFB unit is provided for the purpose of hole transport; and the PFB repeat unit is provided as the emissive unit.
In another example, disclosed in WO 03/095586, it was found that the lifetime of a polymer for use in an optical device, in particular an electroluminescent polymer, may be increased by the incorporation of repeat units that increase the glass temperature (Tg) of the polymer. In particular, incorporation of 2,7-linked 9,9-diarylfluorene repeat units into an electroluminescent polymer, particularly a blue emissive electroluminescent polymer, results in significant increase in that polymer's lifetime. Furthermore, it was found to be unnecessary to have separate hole transporting units and blue emissive units; it was found that both functions may be performed by the PFB unit. Surprisingly, the omission of TFB from the prior art polymers described above was found to result in a significant improvement in lifetime. A preferred embodiment disclosed in WO 03/095586 is a blue electroluminescent polymer prepared in accordance with the process of WO 00/53656 by reaction of 9,9-di-n-octylfluorene-2,7-di(ethylenylboronate) (0.5 equivalents), 2,7-dibromo-9,9-diphenylfluorene (0.35 equivalents) and N,N′-di(4-bromophenyl)-N,N′-di(4-n-butylphenyl)-1,4-diaminobenzene (0.15 equivalents) to give polymer (P1):

In another example, disclosed in WO 04/041902, it was determined that improved electron injection, and therefore improved device performance, may be accomplished by increasing the electron affinity of known polyfluorenes (by providing a deeper LUMO). This was achieved in WO 04/041902 by providing fluorene repeat units having electron withdrawing aryl groups. It was also found that increasing electron affinity in this manner lead to better lifetimes for the polymers disclosed therein.
U.S. Pat. No. 6,309,763, discloses a copolymer comprising 10-90% by weight of the group of formula (y):

wherein R1 is independently selected in each occurrence from, among other things, C1-C20 hydrocarbyl. The repeat unit is provided in a copolymer with 10-90% triarylamine. In all the examples each R1 is C8H17 as in polymers (w) and (P1) discussed above.
EP 1528074 also discloses various polymers comprising the above-identified fluorene group in which R1 is C8H17. In example 2 of EP 1528074, there is also disclosed a polymer comprising a 9,9-dimethylfluorene unit directly bonded to nitrogen atoms in the backbone as shown in formula (y2):

JP 2004-131700 also discloses various polymers comprising a fluorene group in which R1 is C8H17. In addition, this document also discloses a polymer comprising 9,9-dimethylfluorene directly bonded to silicon in the polymer backbone as shown in formula (y3):

It is an aim of the present invention to provide a means for increasing the lifetime of polymers for use in an optical device above that of prior art polymers such as those discussed above. It is a further aim of the invention to provide a long-lived polymer for use in an opto-electrical device, particularly a long-lived blue electroluminescent material. It is a yet further aim of the invention to provide a means for increasing the thermal stability of the prior art polymers such as those discussed above. It is yet a further aim of the invention to provide improved device performance.